[ODE] Regarding CSG

[dword at dword.dk]

> The subtractive CSG thing ocurrs to most ODE users sooner or later (it did
> to me once : ), but there's one fatal flaw: subtraction creates all kinds of
> funky shaped edges - and these edges need to be collided against.  So
> disregarding contacts inside a 'negative' geom is not enough.
>
> I doubt it can be done without triangulating.

I'm just guessing here; correct me if I'm wrong. But couldn't at least
subtraction of e.g. a sphere from a box be done by creating a box geom and an
"inversed" sphere geom (where collision occurs on the inside of the sphere).
Then if another body collides with _both_ the box and the inversed sphere, then
it collides with the CSG geom.

+-----------------+
| inside both box |
| and i-sphere  __|__
|  COLLISION!  /x |  \   outside box/inside i-sphere
+-------------|---+ y |
               \_____/   x: inside box/outside i-sphere
                         y: outside box/outside i-sphere

-Christian Larsen